Method for electrolytically forming tapered or contoured cavities



June 21, 1966 A. WILLIAMS 3,257,300

OIRgIgNG TAPERED METHOD FOR EL 0 ROLYTICALLY F R CONTOURED CAVIT FiledFeb. 2O 1961 2 Sheets-Sheet 1 2 Sheets-Sheet 2 Hill.

I V II June 21, 1966 L. A. WILLIAMS METHOD FOR ELECT CA FORMING TAPERED0R 0 ED ITIES ROLYTI ONTOUR Filed Feb, 20., 1961 HIM.

United States Patent 3,257,300 METHOD FOR ELECTROLYTICALLY FORMINGTAPERED OR CONTOURED CAVITIES Lynn A. Williams, Winnetka, Ill., assignorto Anocut Engineering Company, Chicago, 11]., a corporation of IllinoisFiled Feb. 20, 1961, Ser. No. 90,438 8 Claims. (Cl. 204-143) In someinstances, it is desired to produce cavities in which the side walls arenot straight but, rather, are tapered or contoured,'and it is desirablethat these should be produce with accuracy at high speed, and that theyshould have a high degree of polish or smooth finish.

While it is possible to produce contours in workpieces for ElectrolyticShaping, issued into Patent No, 3,019,-

178, dated January 30, 1962; Serial No. 853,194, filed November 16,l959,'en titled Apparatus for Electrolytic Hole Sinking, issued intoPatent No. 3,120,482, dated February 4, 1964; Serial No. 35,646, filedJune 13, 1960. entitled 'Electrode for Electrolytic Shaping, issued intoPatent No. 3,123,545, dated March 3, 1964; Serial No. 35,647, filed June13, 1960, entitled Electrolytic Cavity Sinking Apparatus and Method,issued into Patent No. 3,196,093, dated July 20, 1965; and Serial No.36,314, filed June 15, 1960, entitled Electrolytic Shaping Apparatus andMethod.

The principal object of this invention is to facilitate the formation ofcavities having tapered or contoured side walls with a smooth and insome instances with a high finish.

Another object is to provide electrodes for the formation of suchcavities.

Another object is to increase the speed of forming such cavities.

Another object is to facilitate the manufacture of suitable toolelectrodes for forming such cavities.

Other objects and advantages will appear from the following descriptiontaken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic view in elevation of the work area of apparatusused in the practice of this invention;

FIG. 2 is a frontal'view showing the working tip of a wedge electrode ofthis invention;

FIG. 3 is a side elevation of the electrode of FIG. 2;

FIG. 4 is an end view of the electrode of FIG. 2;

FIG. 5 is a sectional view of a workpiece showing a preliminary roughingcut;

FIG. 6 is a similar view of the same workpiece shown in FIG. 5 afterfinishing the cut to produce tapered side walls;

FIG. 7 is a side elevation of an electrode for forming a slot as a firststep in the practice of the invention;

FIG. 8 is an end view of the electrode'of FIG. 7;

FIG. 9 is aside elevation of a finishing electrode;

FIG. 10 is an end view of the finishing electrode of FIG. 9; j

FIG. 11 is a sectional view of a workpiece with a roughing slot formedby the electrode of FIGS. 7 and 8;

FIG. 12 is a sectional view of the same workpiece after it has beencontoured by the electrode of FIGS. 9 and 10; and

FIG. 13 is a view, partly in section, of a modified arrangement whereina solid contouring electrode is used with the electrolyte beingintroduced through ahole in the workpiece.

. fastened to a front plate a ram R. The front plate is by the methodsshown in the above identified copending applications and the patentsissued thereon, it is difficult to produce good accuracy and smoothfinish when the surface to be formed deviates more than about 45 to 60degrees from a plane normal to the direction of advance of theelectrode, and this invention is concerned primarily, though notexclusively, with this kind of situation. But, as will be seen, theinvention may also be used to produce very smooth surfaces which aresubstantially normal to the path of advance of the electrode. This isshown, for example, in the electrode of FIGS. 9 and 10 andin theworkpiece of FIG. 12.

In the above identified copending application patents the discovery wasshown that by maintaining the elecrolyte under high pressure and, at thesame time, maintaining high electrolyte velocity in the work gap a highsurface finish could be induced in the work material, and this basicdiscovery is used in this invention. (See FIG. 24 and the relateddescriptive matter of the above identified copending application SerialNo. 772,960, now Patent No. 3,058,895.) [In that application and patent,there was disclosed the production of a high finish by the confinementof the electrolyte by use of an external restriction made of neoprene orrubber sealing rings. In the instant invention the sealing, in effect,is accomplished by the electrode itself, as will more fully appear inthe following description.

Briefly, it has been found that if the electrode is arranged in such away as to cause always a kind of outwardly wedging action against thework material, thereby tending constantly to confine the electrolyte, tomaintain its pressure, and to maintain high velocity, then it ispossible to secure close conformation of shape between the cavity in theworkpiece and that on the electrode. And, at the same time, it ispossible to produce a bright, specular finish even though theelectrolyte solutions used are not of the kind ordinarily regarded ashaving electropolishing characteristics.

Referring to the drawings, FIG. 1 shows schematically the generalconfiguration of the apparatus. An electrode E is mounted to a manifoldM, which is fed with a hose H with electrolyte under a pressure of -100to 300 psi. at the manifold and at the entrance to the electrode. Themanifold M is mounted in turn to a holder H1 which is electricallyinsulated from the body of the ram, and the ram itself is protectedagainst electrolyte by a collapsible boot B. Direct current of a voltagenot appreciably greater than about 18 volts is. supplied from a sourceof the type disclosed in the copending application of Lynn A. Williams,Serial No. 863,246, filed December 31, 1959, entitled Control andOperating System for Electrolytic Hole Sinking, by two cables C and C+,indicating the polarity arranged to make the work an anode; The machineis fitted with a worktable WT on which may be mounted a fixture F, andto this the Work WK is fastened with a clamp or vise. The entire workare is surrounded by an enclosure, which is intended to prevent theundesirable exit of splatter and spray. However, the work enclosureshould be provided with vents and exhausted by a power blower to preventthe accumulation of hydrogen or unpleasant or noxious gases. It shouldbe understood that the ram is arranged to be advanced under a positivedrive at a predetermined fixed rate in the direction of the arnow towardthe Work, thereby moving the electrode linearly into the workpiece. Amore detailed description Patented June 21, 1966 aasasco of the type ofapparatus which may be used is shown in the copending application ofLynn A. Williams entitled Electrolytic Cavity Sinking Apparatus andMethod, Serial No. 73,154, filed September 2, 1960.

FIG. 2 shows one form of electrode used in the practice of theinvention. It consists of an electrode proper 21, which is made ofcopper and is mounted in the manifold member M, which, in turn, may beeither fastened to or made integrally with a holder plate H1. Themanifold M is recessed as shown at 23, and provision is made forconnection of a hose through a screw-threaded opening 25. The electrode21 conforms to the shape of the recess 23 in the manifold, and may befastened into it by any suitable means, for example, by brazing orsoldering. The electrode proper 21 is arranged to feed electrolyte fromthe recess 23 of the manifold to its working tip by a plurality of feedholes or passages 27. These should be drilled as conveniently closetogether as is possible without breaking out from one hole through theland into the next. In an electrode having a working tip with a width ofthe order of A", the holes should be about /8" in diameter, and, ingeneral, the more open and free the passage for electrolyte the better.The holes 27 are not carried through all the way to the working tip ofthe electrode but terminate about A2" to A1 above it, where they areintersected by a transverse slot 29 which is milled into the end of theelectrode in such a way as to communicate with the holes 27. The slotmay be about wide for an electrode of this size. The slot 29 should bedeep enough so as to smoothout the flow from the several holes 27, sothat the electrolyte issues smoothly through the slot with a minimumreflection of the pattern of holes through which it previously passed.

This electrode is not insulated either on its sides or on its ends, butis left bare for the purpose of producing tapered side walls 30. Theelectrode may be used in either of two ways. Either it may be simplyadvanced into a raw piece of work, using the maximum penetration ratewhich is obtainable, or, preferably, it may be used to enlarge a cavitypreviously made by a slotting electrode; for example, one like thatshown in FIGS. 7 and 8 or as shown in one or more of the previouslyidentified copending application. A cut of this preliminary kind isshown in FIG. 5. There is no basic reason why it could not be producedby conventional machining as well as by electrolytic means, althoughordinarily formation of cavities of this sort can be accomplished morequickly by use of electrolytic machining techniques than by conventionalmethods. The width and length of the preliminary cut as shown in FIG.should be about the size of the working tip of the tapered electrodeshown in FIGS. 2, 3 and 4.

The method of advancing the tapered electrode directly into a workpiecewithout making a preliminary cut can be carried out where the taper onthe sides and ends of the electrode is sufficiently great (say, 30 fromthe line of advance of the electrode). As the angle of taper becomesless, a point is reached where it is necessary to make a preliminary cutlike that shown in the workpiece of FIG. 5. This is for the reason thatit is not pos sible to maintain a sufficiently fast rate of frontaladvance into the work to keep the tapered sides sufficiently close tothe work material, which, of course, is being electrolytically removedin a generally sideward direction. By making the preliminary cut asshown in FIG. 5 it becomes possible to advance the electrode at a ratewhich is high enough to keep up with the rate of sideward removal, asthere is no concern with the frontal removal at the tip of theelectrode. In addition, the cavity shown in FIG. 5 aids in spreading theelectrolyte uniformly over the entire side surfaces, for the cavityforms a kind of plenum which quickly fills with electrolyte and,thereafter, induces a uniform flow which is helpful in producingaccurate work. FIG. 6 shows the shape of the tapered cavity formed inthe same workpiece shown in FIG. 5 by using the electrode of FIGS. 2, 3and 4.

It is important that a very copious supply of electrolyte be provided,and, accordingly, the supply hose H should be large enough to deliverthe full volume without substantial loss of pressure, and, similarly,the manifold recess 23 should be adequate in size to assure full fiowand uniform distribution Referring to FIGS. 7, 8, 9 and 10, a somewhatdifferent application of the same invention is shown. FIGS. 7 and 8 showa simple electrode for making a rectangular cavity. The electrode proper21 may be made like other electrodes of copper. It is fitted into arecess 23 in a manifold M which, in turn, is fastened to a holder ormounting plate H1. The cavity 23 is fed by an opening adapted to receivea hose connection as at 25. The electrode proper is drilled with feedholes or passages 27, which are closely spaced as previously describedin connection with the electrode of FIGS. 2, 3 and 4. A slot 29 ismilled into the electrode near its tip, and, then, an additional memberserving as a flange plate 31 is fastened to the body of the electrode bybrazing or soldering. This additional flange plate is arranged toproject beyond the body of the electrode by about .010" to .030 with thepurpose of providing clearance for insulating material 33, which isapplied to the body of the electrode to prevent excessive side action.

After this electrode of FIGS. 7 and 8 has been used to form a cavitylike that shown in section in FIG. 11, then the electrode of FIGS. 9 and10 is used in order to modify the shape of the cavity to provide radiion all of the side walls. This electrode of FIGS. 9 and 10 is made insubstantially the same way as the electrodes previously described hereinwith like reference indicia referring to similar parts. Here, however,it is not necessary to provide any slot, but, instead, quite large feedholes 27 are used, and these pass directly through the body of theelectrode 21 to feed electrolyte into the work area.

Unlike the preceding electrodes, this type is machined to a shape toprovide a full radius as shown at R. This, of course, may be any contourwhich is desired and may be either straight or curved as the workrequirements dictate.

After forming the cavity of FIG. 11, then the electrode of FIGS. 9 and10 is positioned so that the working tip of this electrode registerswith the previously formed cavity. It is not critical that the workingface should have identically the same contour as the cavity, but thedeviation should not be excessive and, particularly important, thereshould not be any place where the cavity is so much larger than thefinishing electrode that any large amount of the electrolyte will escapethrough the gap which would thus be formed. Except for this, however,close conformity is desirable but not necessary.

After positioning the electrode of FIGS. 9 and 10 with respect to thepre-existing cavity, it is then advanced at a fairly rapid rate into thework until it has reached a depth where its working tip is close to thebottom of the previously formed cavity. If this is done rapidly, theresult will be to form an accurate contour and to induce a very highfinish on the work material.

In practice, an electrode like that'of FIGS. 7 and 8 has been made as aroughing electrode, its dimensions being 2" in length and in width. Thecavity like that shown in FIG. 11 was produced with an infeed rate ofadvance of .100 per minute using a direct current of 14 volts and anelectrode pressure of about 200 p.s.i. The cavity in the workpiece wascarried to a depth of .200.

Subsequently, an electrode like that of FIGS. '9 and 10 was applied,using a feed rate of .200" per minute, an electrolyzing current at avoltage of 13 volts and a total penetration to a depth of .187".

Parts Potassium chloride 3 Potassium nitrate 1 Rochelle salts 1Potassium citrate 1 For some kinds of work a solution of about tenpercent of sodium chloride can be used satisfactorily. This isparticularly applicable for ordinary tool steels as distinguished fromthe higher nickel alloys found in stainless steel and high temperaturesuper-alloy materials. For the latter the electrolyte of the precedingparagraph is preferred.

In FIG. 13 there is illustrated an arrangement for forming a tapered orcontoured cavity without using a hollow electrode, The workpiece WK isinitially formed with a through cavity 35 by electrolytic orconventional machining techniques. The workpiece is mounted or clampedin. a fixture or bracket 37 having one or more passages 39 therethroughadapted to be placed in communication with the workpiece cavity 3-5. Ifthe workpiece cavity is large or elongate then two, three or morefixture passages 39 fed from a manifold similar in function to themanifold M of FIGS. 2 to 4, 9 and 10, should be provided.

ply source by the hose H and fitting 41 so that a copious supply ofelectrolyte under pressure may be obtained.

The electrode 43 is similar in configuration (for illustration purposesonly) to that shown in FIGS. 9 and 10 but is made of a solid block ofcopper or other suitable material and is mounted directly upon theholder H1 to be connected into the electrolyzing power circuit in amanner to be cathodic. It is formed with contouring radii R which areadapted electrolytically to form contoured surfaces R" on the workpieceWK.

With the electric current turned on and electrolyte being supplied tothe rough workpiece cavity 35, the electrode 43 is advanced into thework with a positive feed and at a constant rate. The rough cavity inthe workpiece forms a plenum ahead of the advancing workpiece and as theelectrolyte escapes from the cavity between the electrode surfaces R andthe workpiece the latter is electrolytically eroded to the shape of thesurfaces R to have a smooth finish and, depending upon the character ofthe workpiece material, perhaps a high specular finish.

It should be appreciated that the essential difference between the-first two forms of the invention and the last is the direction ofelectrolyte flow. In all three there is formed in the workpiece a kindof plenum to which the electrolyte is fed under high pressure and fromwhich it escapes at high velocity between the active faces of theelectrode and those portions of the workpiece being electrolyticallyeroded.

It has been found practical, for example, to use this method in makinghollow bolts each having a recessed hexagonal socket, like an Allenscrew. This can be done by electrolytically sinking a hole through thelength of the bolt. Electrolyte is then introduced into the bottom ofthe hole and a hexagonally shaped electrode is fed into the head of thebolt cavity to enlarge it and to change its shape from round'tohexagonal.

While three rather simple forms of the invention have been illustratedand described, it should be understood that many variations andcomplications are possible. Thus, for example, while a wedge shapedelectrode is shown in FIGS. 2, 3 and 4 having a rather regular shape,other electrodes have also been used in which, while the same generaltapered configuration was employed, the actual shape was; the rathercomplex shape existing between two blades of a turbine wheel, the bladesthemselves having a twist or camber and, at the same time, a constantlychanging section. In this instance, a preliminary, straightsided cut wasmade into a disc of material at a point roughly representing themidpoint between two of the blades. Subsequently the wedge shapedelectrode formed to produce the interblade cavity was introduced at ahigh rate of feed in order to shape and finish the convex side of oneblade and, concurrently, the adjoining concave side of the next blade.

The essential idea is that the electrode should be one in which itssection grows larger as it is advanced into the work so as to removematerial by side action while at all times forcing the electrode intothe cavity to maintain close spacing between the electrode and the work,a spacing of the order of .015 or less while, at the same time,maintaining electrolyte pressure of the order of psi. or more and, atthe same time, maintaining high velocity of electrolyte flow. The rateof advance necessary to bring about these conditions will, of course, bedetermined by the taper. If the taper is shallow, then the electrode mayhave tobe advanced very rapidly, as rapidly, for example, as an inch perminute or even more. On the other hand, if the taper is steeper then aslower rate of advance will maintain the desired condition. In the caseof the electrode shown in FIGS. 9 and 10, it has been found in practicethat it is not necessary to change the rate of advance even though therate of taper changes. Apparently, the feed rate of .200" per minute isa reasonable comprise between the rate one might use for the very slightdegree of taper at the tip itself and the much greater taperapproaching, in fact, a surface normal to the line of advance at theupper limit of the radius portion. Theoretically, it might be desirableto advance an electrode of this kind at first more rapidly and then moreslowly, but, as has been said, it was not found to be necessary in factin this particular instance.

Many other variations and changes may be made without departing from thespirit and scope of the invention, and the scope of the invention is tobe determined from the scope of the following claims.

What is claimed as new and desired to be secured by United StatesLetters Patent is:

1. In the method of forming cavities having shaped side walls and thelike in a workpiece capable of being electrochemically eroded whereinthe cavity mouth is of appreciably greater dimensions than the base,comprising forming a preliminary generally straight sided cavity inthe'workpiece having transverse shape and dimensions corresponding tobut not greater than those of the base of the finished cavity, advancinga finishing electrode into the cavity a relatively high feed rate of atleast about .100 inch per minute, the electrode having electricallyconductive shaping side walls and electrolyte passages therethrough,impressing a direct current having a voltage not greater than about 18volts between the workpiece and the electrode in a sense to make theworkpiece anodic, feeding electrolyte through the electrode passagesunder a pressure of at least about 100 psi. at the entry to theelectrode and with a high electrolyte velocity so as to fill theworkpiece cavity beyond the working tip of the electrode and to escapefrom the cavity between the cavity walls and the electrode side walls,and maintaining a gap spacing between the electrode side walls and thecavity walls of'not greater than about .015 inch, whereby the cavitywalls are electrolytically eroded to a shape complemental to that of theelectrode and given a smooth finish.

2. In the method of forming cavities having shaped side walls and thelike in a workpiece capable of being electrochemically eroded whereinthe cavity mouth is of appreciably greater dimensions than the base,comprising forming a preliminary generally straight sided cavity in theworkpiece having transverse shape and dimensions corresponding to butnot greater than those of the base i at least about 100 p.s.i. at theentry to the electrode and with a high electrolyte velocity so as tofill the workpiece cavity beyond the forward end of the electrode so asto make the workpiece cavity a plenum chamber from which the electrolyteescapes between the electrode side walls and the cavity side walls, andmaintaining a gap spacing between the electrode side walls and thecavity side walls of not greater than about .015 inch.

3. In the method of forming cavities having shaped side walls and thelike in a workpiece capable of being electrochemically eroded whereinthe cavity mouth is of appreciably greater dimensions than the base,comprising forming a preliminary generally straight sided cavity in theworkpiece having transverse shape and dimensions corresponding to butnot greater than those of the base of the finished cavity, advancing afinishing electrode into the cavity of a feed rate of about .200 inchper minute while impressing a direct current having a voltage of notgreater than about 18 volts between the workpiece and the electrode in asense to make the workpiece anodic, the electrode having electricallyconductive shaping side walls, feeding electrolyte under a pressure ofat least about 100 p.s.-i. at the entry into the cavity and with a highelectrolyte velocity so as to fill the workpiece cavity beyond theforward end of the electrode so as to make the workpiece cavity a plenumchamber from which electrolyte escapes between the electrode side wallsand the cavityside walls, and maintaining a gap spacing of not greaterthan about .015 inch between the electrode side walls and the cavityside walls.

4. In the method of forming cavities having shaped side walls and thelike in a workpiece capable of being electrochemically eroded whereinthe cavity mouth is of appreciably greater dimensions than the base,comprising forming a preliminary generally straight sided cavity in aworkpiece having transverse shape and dimensions corresponding to butnot greater than those of the base of the finished cavity, providing ahollow finishing electrode having a cross section of increasingdimensions away from its forward end and having electrically conductiveshaping side walls, impressing a direct current having a voltage of notgreater than about 18 volts between the workpiece and the electrode in asense to make the workpiece anodic, feeding electrolyte through theelectrode under a pressure of at least about 100 p.s.i. at the entry tothe electrode and with a high electrolyte velocity so as to fill theworkpiece cavity beyond the forward end of the electrode so as to makethe workpiece cavity a plenum chamber, and advancing the electrode intothe workpiece at a rate not greater than about one inch per minute andsuch that with the electrolyte outflow from the cavity between thecavity side walls and the electrode side walls a very small spacingdistance of not greater than about .015 inch will be providedtherebetween as the cavity is enlarged to the desired finished shape.

5. In the method of forming cavities having shaped side walls and thelike in a workpiece capable of being electrochemically eroded whereinthe cavity mouth is of appreciably greater dimensions than the base,comprisingv forming a preliminary generally straight sided cavity in theworkpiece having transverse shape and dimensions corresponding to butnot greater than those of the base of the finished cavity, providing afinishing electrode having a cross section of increasing dimensions awayfrom its forward end and having electrically conductive shaping sidewalls, impressing a direct current having a voltage of not greater thanabout 18 volts between the workpiece and the electrode in a sense tomake the workpiece anodic, feeding electrolyte under a pressure of atleast about p;s.i. at the entry to the cavity and with a highelectrolyte velocity so as to fill the workpiece cavity beyond theforward end of the electrode so as to make the workpiece cavity a plenumchamber, and advancing the electrode into the workpiece at a rate in therange between about .100 and one inch per minute such that with theelectrolyte outflow from the cavity between the cavity side walls andthe electrode side walls a spacing distance of not more than about .015inch will be provided therebetween as the cavity is enlarged to thedesired finished shape.

6. In the method of forming cavities having shaped side walls and thelike in a workpiece capable of being electrochemically eroded whereinthe cavity mouth is of appreciably greater dimensions than the base,comprising forming a preliminary generally straight sided cavity in theworkpiece having transverse shape and dimensions corresponding to butnot greater than those of the base of the finished cavity, advancing afinishing electrode into the cavity at a feed rate of about .200 inchper minute while impressing a direct current having a voltage of notgreater than about 18 volts between the workpiece and the electrode in asense to make the workpiece anodic, the electrode having electricallyconductive shaping side walls, feeding electrolyte under a pressure ofat least 100 pounds per square inch at the entry to' the cavity and witha high electrolyte velocity to the workpiece cavity beyond the forwardend of the electrode so as to fill the cavity and so that electrolyteescapes from the cavity between the cavity walls and the electrode sidewalls with a high velocity whereby the cavity walls are electrolyticallyeroded to the desired shape and given a smooth finish, and maintaining agap spacing of not greater than about .015 inch between the electrodeside walls and the cavity walls.

7. In the method of forming cavities having shaped side walls and thelike in a workpiece capable of being electrolytically eroded wherein thecavity mouth is of appreciably greater dimensions than the base,comprising forming a preliminary generally straight sided cavity in aworkpiece having a transverse shape and dimensions corresponding to butnot greater than those of the base of the finished cavity, advancing afinishing electrode into the cavity at a relatively high feed rate ofnot less than about 100 inch per minute, the electrode havingelectrically conductive cavity shaping side walls, the feed rate beingsuch as to maintain a gap spacing of not more than about .015 inchbetween the electrode side walls and the cavity side walls, impressing adirect current having a voltage of not greater than about 18 voltsbetween the workpiece and the electrode in a sense to make the workpieceanodic, and feeding electrolyte into the cavity ahead of the advancingelectrode under a pressure of at least 100 psi. at the entry to thecavity so as to fill the workpiece cavity beyond the forward end of theelectrode thereby to make the workpiece cavity a plenum chamber, wherebythe electrolyte outflow from the cavity between the cavity side wallsand the electrode side walls will be at a high velocity through the verysmall spacing distance between the electrode and the cavity side wallsas the cavity is enlarged to the desired finished shape.

8. In the method of form-ing cavities having shaped side walls and thelike in a workpiece capable of being electrolytically eroded wherein thecavity mouth is of appreciably greater dimensions than the base,comprising forming a preliminary generally straight sided cavity in aworkpiece having a transverse shape and dimensions corresponding to butnot greater than those of the base of the finished cavity, advancing afinishing electrode into the cavity at a relatively and substantiallyconstant rate of not greater than about one inch per minute, theelectrode having electrically conductive cavity shaping side walls, thefeed rate being such as to maintain a gap spacing of not greater thanabout .015 inch between the electrode side walls and the cavity sidewalls, impressing a direct current having a voltage of not greater thanabout 18 volts between the workpiece and the electrode in a sense tomake the workpiece anodic, feeding electrolyte into the cavity ahead ofthe advancing electrode under a pressure in excess of 100 pounds persquare inch at the entry to the cavity so as to fill the workpiececavity beyond the forward end of the electrode thereby to make theworkpiece cavity a plenum chamber, whereby the electrolyte outflow fromthe cavity between the cavity side walls and the electrode side wallswill be at a high velocity through the very small spacing distancebetween the electrode and the cavity side walls as the cavity isenlarged to the desired finished shape.

References Cited by the Examiner UNITED STATES PATENTS 178,448 -6/187'6Laughlin 77-66 1,435,671 11/1922 Stewart 204-280 1,707,406 4/ 1929Miguet 204280 10 2,362,260 11/ 1944 Foster 77-66 2,786,373 3/1957 Patton77-68 2,813,966 11/1957 Matulaitis 219-69 2,818,490 12/1957 Dixon 219-692,835,198 5/1958 Engle 204-143 2,844,531 7/ 195 8 Prince 204-1432,848,401 8/1958 Hartley 204-143 2,909,641 10/ 1959 Kucyn 20'4-1432,982,842 5/ 1961 Tuscher 219-69 3,019,178 1/1962 Williams 204-1433,058,895 10/ 1962 Williams 204-143 3,095,364 6/1963 Faust et a1 204-1433,120,482 2/ 1964 Williams 204-143 FOREIGN PATENTS 335,003 9/1930 GreatBritain.

OTHER REFERENCES Faust et a1. (B), Iron Age, pp. 77-80, Nov. 3, 1960.

Gusev et 'al., ZAUKA I SHIZN', No. 12 (Electrical Working for Metals,excerpted from pages 25 and 27), 1954.

JOHN H. MACK, Primary Examiner.

A. D. SULLIVAN, R. L. GOOCH, Assistant Examiners.

1. IN THE METHOD OF FORMING CAVITIES HAVING SHAPED SIDE WALLS AND THELIKE IN A WORKPIECE CAPABLE OF BEING ELECTROCHEMICALLY ERODED WHEREINTHE CAVITY MOUTH IS OF APPRECIABLY GREATER DIMENSIONS THAN THE BASE,COMPRISING FORMING A PRELIMINARY GENERALLY STRAIGHT SIDED CAVITY IN THEWORKPIECE HAVING TRANSVERSE SHAPE AND DIMENSIONS CORRESPONDING TO BUTNOT GREATER THAN THOSE OF THE BASE OF THE FINISHED CAVITY, ADVANCING AFINISHING ELECTRODE INTO THE CAVITY A RELATIVELY HIGH FEED RATE OF ATLEAST ABOUT .100 INCH PER MINUTE, THE ELECTRODE HAVING ELECTRICALLYCONDUCTIVE SHAPING SIDE WALLS AND ELECTROLYTE PASSAGES THERETHROUGH,IMPRESSING A DIRECT CURRENT HAVING A VOLTAGE NOT GREATER THAN ABOUT 18VOLTS BETWEEN THE WORKPIECE AND THE ELECTRODE IN A SENSE TO MAKE THEWORKPIECE ANODIC, FEEDING ELECTROLYTE THROUGH THE ELECTRODE PASSAGESUNDER A PRESSURE OF AT LEAST ABOUT 100 P.S.I. AT THE ENTRY TO THEELECTRODE AND WITH A HIGH ELECTROLYTE VELOCITY SO AS TO FILL THEWORKPIECE CAVITY BEYOND THE WORKING TIP OF THE ELECTRODE AND TO ESCAPEFROM THE CAVITY BETWEEN THE CAVITY WALLS AND THE ELECTRODE SIDE WALLS,AND MAINTAINING A GAP SPACING BETWEEN THE ELECTRODE SIDE WALLS AND THECAVITY WALLS OF NOT GREATER THAN ABOUT .015 INCH, WHEREBY THE CAVITYWALLS ARE ELECTROLYTICALLY ERODED TO A SHAPE COMPLEMENTAL TO THAT OF THEELECTRODE AND GIVEN A SMOOTH FINISH.